<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>demo07</title>
</head>
<body>
  <canvas width = "300" height = "300" id = "my_Canvas"></canvas>

      <script>
         /*============= Creating a canvas ==================*/
         var canvas = document.getElementById('my_Canvas');
         gl = canvas.getContext('experimental-webgl');
         
         /*========== Defining and storing the geometry ==========*/

         var vertices = [
            -0.5,0.5,0.0,
            -0.5,-0.5,0.0,
            0.5,-0.5,0.0,
            0.5,0.5,0.0
         ];

         var colors = [0,0,1, 1,0,0, 0,1,0, 1,0,1,];
         
         indices = [3,2,1,3,1,0];
         
         // Create an empty buffer object and store vertex data
         var vertex_buffer = gl.createBuffer();
         gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);
         gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
         gl.bindBuffer(gl.ARRAY_BUFFER, null);

         // Create an empty buffer object and store Index data
         var Index_Buffer = gl.createBuffer();
         gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);
         gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(indices), gl.STATIC_DRAW);
         gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);

         // Create an empty buffer object and store color data
         var color_buffer = gl.createBuffer ();
         gl.bindBuffer(gl.ARRAY_BUFFER, color_buffer);
         gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(colors), gl.STATIC_DRAW);

         /*======================= Shaders =======================*/
         
         // vertex shader source code
         var vertCode = 'attribute vec3 coordinates;'+
            'attribute vec3 color;'+
            'varying vec3 vColor;'+
            'void main(void) {' +
               ' gl_Position = vec4(coordinates, 1.0);' +
               'vColor = color;'+
            '}';
            
         // Create a vertex shader object
         var vertShader = gl.createShader(gl.VERTEX_SHADER);

         // Attach vertex shader source code
         gl.shaderSource(vertShader, vertCode);

         // Compile the vertex shader
         gl.compileShader(vertShader);


         // fragment shader source code
         var fragCode = 'precision mediump float;'+
            'varying vec3 vColor;'+
            'void main(void) {'+
               'gl_FragColor = vec4(vColor, 1.);'+
            '}';
            
         // Create fragment shader object
         var fragShader = gl.createShader(gl.FRAGMENT_SHADER);

         // Attach fragment shader source code
         gl.shaderSource(fragShader, fragCode);

         // Compile the fragmentt shader
         gl.compileShader(fragShader);

         // Create a shader program object to
         // store the combined shader program
         var shaderProgram = gl.createProgram();

         // Attach a vertex shader
         gl.attachShader(shaderProgram, vertShader);

         // Attach a fragment shader
         gl.attachShader(shaderProgram, fragShader);

         // Link both the programs
         gl.linkProgram(shaderProgram);

         // Use the combined shader program object
         gl.useProgram(shaderProgram);

         /* ======== Associating shaders to buffer objects =======*/

         // Bind vertex buffer object
         gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);

         // Bind index buffer object
         gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);

         // Get the attribute location
         var coord = gl.getAttribLocation(shaderProgram, "coordinates");

         // point an attribute to the currently bound VBO
         gl.vertexAttribPointer(coord, 3, gl.FLOAT, false, 0, 0);

         // Enable the attribute
         gl.enableVertexAttribArray(coord);

         // bind the color buffer
         gl.bindBuffer(gl.ARRAY_BUFFER, color_buffer);
         
         // get the attribute location
         var color = gl.getAttribLocation(shaderProgram, "color");
 
         // point attribute to the volor buffer object
         gl.vertexAttribPointer(color, 3, gl.FLOAT, false,0,0) ;
 
         // enable the color attribute
         gl.enableVertexAttribArray(color);

         /*============Drawing the Quad====================*/

         // Clear the canvas
         gl.clearColor(0.5, 0.5, 0.5, 0.9);

         // Enable the depth test
         gl.enable(gl.DEPTH_TEST);

         // Clear the color buffer bit
         gl.clear(gl.COLOR_BUFFER_BIT);

         // Set the view port
         gl.viewport(0,0,canvas.width,canvas.height);

         //Draw the triangle
         gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);
      </script>
</body>
</html>